Half life implies that one half of the mehane molecules decay in that period (or that a single one has a 50% of decaying).

Depends on what 'average' you mean I guess. It is true for the median of course. However, if the time for an individual molecule to decay is exponentially distributed, you can solve for lambda to find its expected lifetime is 13.1 years. That being said, I'd say this is a less useful interpretation.